1. Field of the Invention
This invention relates to a photodetecting device and a method of manufacturing the same, and more particularly to a photodetecting device having quantum dots and a method of manufacturing the same.
2. Description of the Related Art
Conventionally, there has been used a photodetecting device that uses a quantum well structure having quantum well layers in a photodetecting section thereof (see e.g. Japanese Unexamined Patent Publication No. 2000-323742).
The photodetecting device has become capable of detecting infrared light having a wavelength of 10 μm or so, which had been considered difficult to detect, by using the quantum well layers made of e.g. a gallium arsenide (GaAs)-based semiconductor material.
However, the photodetecting device employing the quantum well structure does not detect light which is incident perpendicularly to the layer surfaces of the quantum well layers. To solve this problem, the photodetecting device was required to be provided with a contrivance, such as a special optical device or diffraction grating, for making the incident light substantially parallel to the layer surfaces of the quantum well layers. This resulted in the complicated construction of the photodetecting device, and increased manufacturing costs thereof. On the other hand, the photodetecting device employing the quantum well structure suffered from the problem that dark current, which can be a noise source, is exponentially increased with respect to the operating temperature of the photodetecting device. Therefore, it is difficult for the photodetecting device to operate at a temperature of 300K, which is essential in putting the device to use, and hence it was necessary to cool the device to a temperature at which the adverse influence of the dark current can be made negligible. This also brought about the problem of increased cooling costs of the device.
To cope with these problems, a photodetecting device has been proposed which employs a quantum dot structure having quantum dots in place of the quantum well structure (e.g. see Semiconductor Science and Technology, 1996, Vol. 11, pp. 759 to 765).
The photodetecting device employing the quantum dot structure is capable of detecting light which is incident perpendicularly to the layer surfaces of the quantum dot structure of a photodetecting section thereof. Further, the probability that photoexcited carriers are captured again by the quantum dots is low, and hence it is expected that the photodetecting device can have a high photocurrent gain to achieve high sensitivity. Furthermore, if the quantum dot structure is used in the photodetecting section, it is expected that the signal current can be detected without being buried in the dark current even at a relatively high temperature. This makes it possible to simplify a cooling device for the quantum dot structure for controlling the dark current, and hence the downsizing of the photodetecting device and the reduction of cooling costs thereof can be expected.
However, when the photodetecting device employing the quantum dot structure is actually manufactured, the sensitivity of photodetection is suddenly degraded along with a rise in the temperature of the photodetecting section.
One of the causes of this degradation is the lowered probability of emission of electrons caused by the rise in the temperature of the photodetecting section. Hereinafter, a description will be given of the mechanism causing the lowering of the emission probability.
FIG. 5 is a schematic cross-sectional view of essential parts of a photodetecting device employing a quantum dot structure. FIG. 6 is a schematic diagram of the potential distribution of a conduction band of the photodetecting device shown in FIG. 5. The photodetecting device 100 shown in FIG. 5 comprises a substrate 101, a single embedding layer 102 formed over the substrate 101, and a quantum dot structure 105 which is formed over the single embedding layer 102 and includes quantum dots 105a and a single embedding layer 106 arranged in the mentioned order. FIG. 6 schematically shows the potential distribution of the conduction band of the photodetecting device with respect to the Fermi level taken on line X-X′ of FIG. 5, in which are indicated the potential energy 105b of the quantum dots 105a, the Fermi level (Ef), a conduction band end potential (Ec1) at low temperature, and a conduction band end potential (Ec2) at high temperature.
Generally, a potential barrier Ec−Ef, which is the difference between a conduction band end potential (Ec) and the Fermi level Ef, and is produced by a semiconductor layer, can be expressed based on the charge neutral condition by the following equation (1):Ec−Ef=kbT×ln(Nc/Nd)  (1)
In the this equation (1), kb represents a Boltzmann constant, T the temperature of a photodetecting section 100a, Nc an effective density of state of the conduction band of the embedding layers 102 and 106, and Nd an impurity concentration.
As can be understood from the equation (1), the potential barrier Ec−Ef becomes larger in proportion to the temperature of the photodetecting section 100a. More specifically, it is apparent from FIG. 6 that the potential barrier in the vicinity of the quantum dot structure 105 becomes larger in proportion to a rise in the temperature of the photodetecting section 100a (Ec2−Ef>Ec1−Ef).
For this reason, in a process during which light energy is obtained from light absorbed by the quantum dot structure 105 of the photodetecting section 100a, and excited electrons are emitted as photocurrent, when the temperature of the photodetecting section 100a becomes higher, the potential barrier also becomes larger, and therefore the probability of emission of electrons becomes lower. This results in a sudden degradation of the sensitivity of photodetection by the photodetecting device 100.